Experimental Study on CO2 Methanation over Ni/Al2O3, Ru/Al2O3, and Ru-Ni/Al2O3 Catalysts
Round 1
Reviewer 1 Report
In this manuscript, Chein and Wang studied CO2 Methanation over Ni/Al2O3, Ru/Al2O3, and Ru-Ni/Al2O3 catalysts by investigating the metal loading and reaction temperature effects on CO2 conversion, H2 efficiency, CH4 yield, CO yield, and finally the thermal stability. Then the authors showed that the CO2 methanation performance at low temperatures can be enhanced by using the bimetallic Ru-Ni catalyst compared with the monometallic Ru or Ni catalyst.
Overall, the results are clearly presented and contain relevant information for further studies. I have a few suggestions for the authors,
1) Please provide the FE-SEMEDX micrographs of fresh Ru-Ni bimetallic catalyst since that is the best performing catalyst.
2) What is the reasoning for extra noise in Figure 3 XRD pattern of (c) 1wt%Ru-10wt% Ni/Al2O3 catalyst compared to (a) and (b).
3) What is the average particle size of these catalysts?
4) How does the authors make sure that the reaction is not mass transfer limited? Determining apparent activation energy using the Arrhenius plot and comparing it with literature would be useful.
5) The claim that Ru-Ni catalyst showed good thermal stability should be further validated with at least 1 week of testing.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript entitled “Experimental Study on CO2 Methanation over Ni/Al2O3, Ru/Al2O3 and Ru-Ni/Al2O3 catalysts” investigates the application of different catalysts in a wide range of temperatures. I would suggest the authors to publish the work if these major issues were justified.
1.- Regarding the experimental system conditions, could you please justify in the text:
the particle size that is high for homogeneous impregnation of metals and it is also high for inner diffusion of reactants and products during methanation.
The low space velocity used of 5835 h-1 that usually is closer to 20000 h-1. At what temperature did you calculate it? The diameter of the reactor is 4 mm, inner or outter diameter? The high percentage of N2 in the reactant mixture, close to 50 % vol., could affect the conversion obtained? Why did you choose a ratio of reactants higher than stoichiometric? The variations of flow in the methanation reaction are very high, how did you measure the flow out of the reactor?
The length of the catalyst bed is high, did you study the temperature profile in it? How and where do you measure the temperature of the catalyst bed? This is a crucial factor related to a highly exothermal reaction like mehanation. How stable was the temperature during the reaction?
A lot of results used in this study are not shown in the paper and they should be compiled in the supplementary information file. FE-SEMEDX of samples, XRD, results like Figure 5 for all samples studied.
2.- Regarding treatment of data:
In this manuscript, the authors collected data of CO2 conversion, H2 efficiency, CH4 yield and CO yield. CO2 conversion and H2 efficiency are related taking into account the high ratio of H2 used. However, selectivity in heterogeneous catalysis is critical for the design of catalytic processes that minimize the production of undesired byproducts. The authors should introduce this parameter and study the effects of metal loading and temperature reaction on it.
Results and discussion
Ni loading effect
Line 253 “The reason of … bigger crystallite size”. You can introduce the crystallite size calculated with Scherrer equation from the XRD that you performed to the samples or you should perform.
Equations 3 and 10 should have the same number of enthalpy.
Authors introduce different reactions that could influence the overall process of methanation like CH4 decomposition (11) and Boudouard (12). The carbon balance should be introduced to study the influence of these reactions.
Ru-Ni loading effect
Line 408. How do you identify the Ru segregation tendency in your sample?
Line 412. The sentence “It is also seen …high-temperature regime” is confusing.
Stability test
Line 460. “CO2 conversion changes from 80 % to 77%...” Therefore, the maximum conversion at 400 ºC is 80 %. However, in figure 8, the maximum conversion at 400 ºC is 88%. Could you please explain this difference? Could you please introduce calculations of reproducibility of the system? How does influence the 5% accuracy of chemical composition of catalysts?
Line 465. “Since Ru has better resistance …” The authors did not include the stability study of the Ni catalysts nor the Ru catalyst. Therefore, the role of Ru enhancing stability is difficult to justify. In this manuscript, the introduction of Ru in the catalyst produced high CO2 conversion and CH4 yield depending on the temperature, why do you include reference 39?.
Conclusion
- Why the system decrease conversion at 450 ºC compare to 400 ºC if it did not reach equilibrium yet?
- Ru segregation has not been properly justify in this work
- The differences in the stability of CO2 conversion are higher if the first results compiled in figure 8 are used.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
This paper clearly presents a well established experimental study on the CO2 methanation performed on Al2O3 supported Ni, Ru, and Ru-Ni catalysts. Both the form of the presentation and the catalytic details revealed in the manuscript suggest a high level science orinted to practical catalytic issues.
Author Response
Thanks for the positive response from the reviewer.
Reviewer 4 Report
The manuscript “Experimental Study on CO2 Methanation over 3 Ni/Al2O3, Ru/Al2O3, and Ru-Ni/Al2O3 Catalysts” depicts an interesting work on the methanation of CO2 with different catalysts. The catalysts are characterized with XRD and evaluated towards their methanation performance. The methanation performance of the different catalysts is evaluated with increasing temperature and the catalysts stability is tested.
The manuscript is well structured and discussed.
While I like the general outline and the concept of the manuscript, I have a few minor issues which should be addressed:
Introduction:
Line 44: Eckle et al. [7] and Fisher et al. [8]
The references are mixed up.
Characterization:
The information on the SEM and on the XRD device are missing (which manufacturer, which device). For the XRD it would be important to also add the type of the X-ray source. Please provide these experimental details.
Furthermore, the results (X-ray diffractograms and the composition of the catalysts) should be included in the results and not the experimental section. Only the experimental details should be considered in the experimental section. The characterization of the used catalyst is already a new result of your study.
SI: No letters are assigned to the graphs, while a, b, c and d are referred to in the figure captions. The letters should be added.
(S3 letter a is missing in the caption)
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Accept in the present form.
Author Response
Thanks for the positive reply from the reviewer.
Reviewer 2 Report
In this review of the manuscript entitled "Experimental Study on CO2 Methanation over Ni/Al2O3, Ru/Al2O3 and Ru-Ni/Al2O3 catalysts" the authors did not provide a proper answer to the fact that including N2 in the reactant mixture in a high concentration will affect the results obtained since the concentration of reactants is greatly changed.
In addition, they did not introduce important points raised in the first review like the need of studying the selectivity to CH4 in the tests, crystallite size of samples studied, a systematic study of the carbon balance, the study of Ru segregation in the samples of the manuscript, the study of reproducibility of the system or the study of a Ni-catalyst stability.
Because of the lack of a proper justification of all these points I can not support the publication of this manuscript in the actual form
Author Response
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Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
In this second review of the manuscript entitled "Experimental Study on CO2 Methanation over Ni/Al2O3, Ru/Al2O3 and Ru-Ni/Al2O3 catalysts" the authors show themselves redundant in the answers provided previously with no improvment of the points raised to justify rejection.
The authors have not proven that the introduction of N2 does not affect the behaviour of the catalyst.
The authors didn’t introduce important points raised in the first review like the need of studying the selectivity to CH4 in the tests. They answer “In the supplementary information, time on stream measured CH4 and CO selectivities as functions of the catalyst used and reaction temperature corresponding to averaged results shown in Figures 7, 8, 9, and 10 are provided.” However, I could not see them in the supplementary file I have access to.
The authors didn’t prove the increase in the crystallite size with higher metal loading in the samples of this study.
The authors only provide the carbon balance in one set of conditions instead of performing a systematic study of it as suggested.
The authors didn’t prove the segregation of Ru-Ni in the samples of this study, they didn’t identify it to raise this fact as a conclusion of this study.
The authors did not include the stability study of the Ni catalysts nor the Ru catalysts. Therefore, the role of Ru enhancing stability of Ni Catalyst is not justified.
Author Response
please see the attachment.
Author Response File: Author Response.pdf
